1. Field of the Invention
This invention relates to a method of cleaning reticles for use in conjunction with the fabrication of semiconductor devices.
2. Brief Description of the Prior Art
In the manufacture of semiconductor devices, a plurality of reticles are generally required with each reticle providing the basic pattern for each level of the semiconductor device. A problem generally arises whereby contamination or particles are deposited on the reticle and thereby cause a pattern somewhat different from the intended pattern to be formed on the semiconductor device. In fact, with the continued miniaturization of semiconductor components in integrated circuits, often a speck of dirt from the air which may be too small to be noticed can land on the reticle and cause a fatal defect in the finally fabricated device such as, for example, a short circuit or an open circuit.
In order to avoid the above described problem, the reticles have been stored in boxes. When required, the reticle was removed from the box and inspected under very bright light to determine whether specks of dirt were contained on the reticle. The dirt was then blown off of the reticle using a nitrogen gun. This involved holding the reticle and attempting to maintain the reticle clean while also blowing off any dirt contained on the reticle. The reticle was then replaced in the box. An additional optional step included cleaning out of the box with the nitrogen gun to minimize the possibility of contamination of the reticle within the box itself. The reticles were then stored in the boxes in a reticle library which is a closed clean container. When required to be used, the reticles are removed from the boxes in the reticle library, entered into a stepper where they are used to form one or more patterns during processing of a semiconductor device and are then returned to the box in the reticle library.
It is apparent that it desirable to maintain the reticles as clean as possible, with cleanliness being enhanced by removal of the human handling factor to the greatest extent possible.
The above-described problem of the prior art is minimized and there is provided a system and process for in-situ cleaning of the reticles by blow cleaning the reticles in an apparatus designed for such cleaning without the human handling element being involved.
Briefly, the above is accomplished by moving the single reticle to be used from the reticle library using automated equipment as in the prior art for reticle movement to a reticle blow-off chamber. The reticle blow-off chamber is attached to the reticle library in a manner to restrict the entry of any contaminants from the exterior into the chamber. A gas which is inert to the materials on the reticle, preferably nitrogen, is then pulsed, preferably tangentially to the reticle surfaces and over the surface of the reticle to an exhaust. The gas jet through which the gas is injected into the blow-off chamber is designed to force the gas along the surface of the reticle along the entire length of the reticle and on both sides of the reticle. Jet configurations can be, for example, a single jet along the entire length of the reticle with a width sufficient to inject air on both sides of the reticle or two such jets, each jet designed to inject air against one of the sides of the reticle. Another jet configuration can be to provide a plurality of jets along the entire upper edge of the reticle with each such jet either injecting gas on both sides of the reticle or with each jet being split in half or being two jets so that each half injects gas on only one side of the reticle.
It has been determined in accordance with the present invention that the removal of dirt from the surface of the reticle is materially enhanced by a rapid pulsing action of the gas and that a rapid pulsing action, from about 0.5 second up to about 1 second pulse length. The pressure of the gas is from about 20 psi to about 120 psi and preferably about 80 psi and is sufficient to dislodge the dirt from the reticle with great speed and efficiency. The pulse repetition rate of the applied gas is from about 0.5/second to about 40/second. The gas is preferably ionized.
The cleaning gas, which is preferably nitrogen, is filtered to remove any particles therein and is then ionized prior to entry into the blow-off chamber to prevent static charge from being accumulated on the reticle. Static charge would cause the reticle to attract new contamination from the environment. A valve or other appropriate mechanism to provide a pulsing action to the gas entering the blow-off chamber is provided. The valve is controllable externally to allow for changes and control of pulse length as well as the interval between pulses. The valve or other appropriate mechanism is provided to control the pressure of the gas applied to the reticle being cleaned. A typical ionization source uses alpha particles to electrically neutralize the injected nitrogen stream. Ionization can take place anywhere within the nitrogen source line up to entry into the blow-off chamber or within the blow-off chamber by means of injection of ions within the box adjacent to the injected nitrogen stream and preferably taking place immediately before entry into the blow-off chamber or within the chamber at the point of injection.
The blow-off chamber preferably has a groove for holding the reticle in a vertical position and a series of nitrogen jets in the top of the blow-off chamber to pulse the gas in a direction along or tangential to the surfaces of the reticle. An exhaust for the gas with any particles dislodged from the reticle is positioned at the bottom of the blow-off chamber. An in situ particle monitor (ISPM) can be located in the exhaust to monitor the existence of particles in the exhaust. The ISPM shines a laser across the path that particles would take in the exhaust and scatters light whenever the laser beam strikes a particle. A light detector is positioned normal to the path of the laser beam and therefore only receives light when there is a scattering of the light from the laser beam which indicates that particles are still present in the exhaust and that the reticle is not fully cleaned. Cleaning therefore continues until no particles are sensed for some predetermined period of time or until the particle count per unit of time is below some predetermined value, depending on the degree of cleaning required.
After the reticle is cleaned in the blow-off chamber by virtue of the ISPM having reached a count of zero or other predetermined minimum value, the reticle is then automatically positioned in the stepper using the automated equipment of the prior art and the reticle is returned to the reticle library for reuse after its task within the stepper has been completed.
As an alternative, the ISPM can be used periodically by being inserted into one cleaning line on a continuous or part time basis to determine the optimum cleaning time, which presently appears to be about 10 to about 20 seconds. This time can be used and updated in all systems without using the ISPM on a continual basis and/or in all lines.
It can be seen that there has been provided a system and procedure for cleaning reticles which insures that the reticle is cleaned immediately prior to each use. Such cleaning is accomplished in a clean environment and without the intrusion of a human operator in the cleaning environment.